The present invention relates to sources of electromagnetic radiation (EMR) that can produce EMR in the extreme ultraviolet (soft X-ray) range of the electromagnetic spectrum. EMR from such a source can be used for microlithography, which is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like.
As noted above, a key technique in the manufacture of microelectronic devices such as integrated circuits is microlithography. Most conventional microlithography is performed using deep ultraviolet (DUV) light. The pattern to be transferred is defined on a reticle or mask that is illuminated by a beam of DUV light. A downstream image of the illuminated portion of the reticle is projected (usually with demagnification) by a beam of DUV light onto a suitable substrate (e.g., semiconductor wafer) coated with a resist that is xe2x80x9csensitivexe2x80x9d to exposure by the DUV light. Microlithography performed using DUV light still is within the realm of xe2x80x9coptical microlithography.xe2x80x9d
With ever-increasing miniaturization and density of microelectronic devices, the need has become acute for a microlithography method offering greater resolution than optical microlithography. In fact, optical microlithography now is being conducted at or very nearly at the diffraction limit of DUV light, which means that substantially greater resolution than currently obtainable is probably not possible with optical microlithography. As a result of this dilemma, considerable research and development effort currently is underway to develop a practical xe2x80x9cnext generationxe2x80x9d microlithography apparatus. Among top contenders are charged-particle-beam microlithography and xe2x80x9cextreme ultravioletxe2x80x9d (also termed xe2x80x9cEUVxe2x80x9d or xe2x80x9csoft X-rayxe2x80x9d) microlithography. The EUV wavelength range receiving the most current attention is 11 to 13 nm.
Unfortunately, EUV light and EMR of neighboring wavelengths are strongly absorbed by most known substances, and no optical materials are currently known that are transmissive to such EMR. Hence, with such EMR as used for microlithography, there is no known way in which to provide a refracting system that can be used for reticle illumination and/or projection of an image onto a substrate. Consequently, illumination-optical systems and projection-optical systems for use in microlithography performed using these short-wavelength EMRs must be made of reflecting optical elements.
Another difficulty with EUV radiation and related short-wavelength EMR is that reflectance of such radiation from ordinary reflective mirrors is extremely low. To obtain maximal reflectance, the mirrors are configured with reflecting surfaces made of a multilayer-film structure. For example, EUV-reflective mirrors have been produced with multilayer reflective films of molybdenum (Mo) and silicon (Si) for reflecting 13-nm EUV light, and multilayer reflective films of Mo and beryllium (Be) for reflecting 11-nm EUV light. However, even with the most efficient mirrors of these types, reflectance of EUV light is at most about 70%. The resulting loss of EMR at each mirror in the illumination-optical system and projection-optical system has led to considerable difficulty in achieving satisfactory imaging performance and throughput.
EUV radiation used in the technologies summarized above typically is produced from a highly specialized source such as an undulator, a laser-plasma source, or a discharge-plasma source. The latter two are attractive because of their relatively small size. In a laser-plasma source, a high-intensity pulsed laser light is converged on a target material to cause the target material to produce a high-temperature plasma from which EUV radiation is emitted. In a discharge-plasma source, the plasma is produced by electrical discharge between electrodes.
An exemplary plasma-focused source (a type of discharge-plasma source) is disclosed in Japan Kxc3x4kai Patent document no. Hei 10-319195 and shown in FIG. 8. The source includes an anode 1, a cathode 2, and a base member 3 situated inside a vacuum chamber 8. The electrodes 1, 2 are connected to and energized by high-voltage pulses produced by a pulse generator 7. A working-gas mixture (consisting of a buffer gas and a working gas that produces a desired transition when exposed to an electrical discharge) is introduced into the vacuum chamber 8 via a conduit 10. Specifically, the working-gas mixture is introduced by the conduit 10 to a space above the base member 3 and between the anode 1 and cathode 2. The cathode 2 surrounds the anode 1 in the manner of a cylinder. High-voltage pulses from the high-voltage pulse generator 7 are applied across the electrodes 1, 2 to create a discharge between the electrodes 1, 2. The discharge begins on the surface of the base member 3 and produces an xe2x80x9cinitialxe2x80x9d plasma. The initial plasma is formed by ionization of the working gas in the region between the electrodes 1, 2 and above the base member 3.
Upon creation of the initial plasma, electrons and ions in the initial plasma move relative to each other under the influence of the electric-field produced by the voltage gradient between the electrodes 1, 2, thereby forming a current in the plasma. The current in the plasma, in turn, generates a magnetic field in the plasma. The ions and electrons moving through the plasma interact with the magnetic field and move upward. As a result, the plasma becomes concentrated at the distal end of the anode 1. The concentrated plasma has elevated temperature and density, sufficient to produce EUV light that radiates from the plasma.
In these sources, the material that actually forms the plasma is material situated at the electrode member excited by the concentrated plasma. Typically, the material includes not only the electrode member itself but also molecules of the working gas situated in the immediate vicinity of the electrode. The wavelength of EMR produced by the plasma corresponds to specific transitions in ions of the electrode member and of the working gas. The plasma region in which the desired EMR is produced is situated substantially within a volume having a diameter of about 1 mm at the distal tip of the electrode 1. Because plasma production is pulsatile, release of radiation from the plasma is pulsatile. Each pulse of released EMR has a duration in the range of about 0.1 xcexcs to 1 xcexcs. By way of example, if the working-gas mixture surrounding the distal end of the electrode 1 contains lithium vapor, then the resulting line spectrum of the produced EUV radiation is about 13.5 nm, which is attributable to the transition in the lithium ions in the plasma.
The amount of EMR produced per pulse by the plasma-focused source of FIG. 8 is greater than from a laser-plasma light source. Also, with this plasma-focused source, EMR can be produced having a relatively high pulse rate, e.g., of up to several kilohertz. Increasing the pulse rate yields an increase in the net amount of EMR that can be obtained from the source and allows the amount of radiation produced per unit time from the source to be controlled with higher precision.
Japan Kxc3x4kai Patent Document No. Hei 11-312638 discloses use of an EUV light source, as described above, in an EUV microlithography apparatus. The optical system downstream of the source is depicted in FIG. 9 herein, wherein the rays 6 are propagating from the source. The optical elements 11a and 11b are xe2x80x9cfly-eyexe2x80x9d (compound) mirrors having respective surfaces such as shown in FIGS. 10(A) or FIG. 10(B). Upstream of the mirrors 11a, 11b are other mirrors that collect and collimate the EUV radiation produced by the source. Further with respect to FIG. 9, item 12 is a reflective reticle, item 13 is a reticle stage, items 14a-14f are mirrors, item 15 is the substrate, and item 16 is a wafer stage. The mirrors 14a, 14b, along with the mirrors 11a, 11b and mirrors situated between the mirror 11b and the source, constitute the xe2x80x9cillumination-optical system.xe2x80x9d The mirrors 14c-14f constitute the xe2x80x9cprojection-optical systemxe2x80x9d that projects a reduced (demagnified) image of the illuminated portion of the reticle 12 onto the substrate 15.
The maximal achievable reflectance of each of the multilayer mirrors used in the illumination and projection systems is about 70%. In other words, at least about 30% of incident EMR on each mirror is lost. Consequently, after reflection from multiple mirrors to produce the demagnified images at the substrate 15, the maximal amount of EMR initially produced that actually participates in making an exposure on the substrate 15 is only a few percent. Since throughput is a function of the intensity of exposure light, to obtain more rapid exposure and correspondingly improved throughput, every bit of the EMR generated from the source must be gathered and utilized for exposure.
With an illumination-optical system configured as shown in FIG. 9, the respective areas of the reticle 12 and substrate 15 undergoing illumination and imaging, respectively, receive uniform illumination intensity. This is due in part to the uniformizing effects of the mirrors 11a, 11b (FIGS. 10(A) and 10(B)). As a result, imaging performance tends to be independent of the position or direction of the elements of the pattern being projected from the reticle 12 to the substrate 15. For even better imaging performance, it is desirable that the intensity of the EMR flux incident on the mirror 11b have a rotationally symmetrical (relative to the optical axis) distribution of intensity.
However, whenever a plasma-focused light source such as shown in FIG. 8 is used as a source of short-wavelength EMR, substantial limitations are imposed on the configuration of the illumination-optical system. As a result, it is very difficult to form an EMR flux, for illumination purposes, having a rotationally symmetrical intensity distribution with respect to the optical axis. I.e., from a plasma-focused source, the generated EMR propagates radially outward from the plasma. To be useful for microlithographic illumination purposes, the EMR flux 6 from the source must be collimated, as shown in FIG. 9. One possible way in which the EMR from the source can be collimated is to place a mirror, configured as a paraboloid of revolution having a focal point, relative to the source such that the EMR-producing plasma is at the focal point of the mirror. Hence, EMR produced by the plasma reflects from the mirror as a collimated beam. Unfortunately, in conventional configurations of this nature that have been considered to date, the electrodes of the plasma-focused source undesirably block propagation of some of the EMR reflected from the mirror. This blocking limits the solid angle at which the EMR can be utilized from the source and used to form the collimated beam.
Therefore, there is a need for improved devices and methods for forming a collimated flux of short-wavelength EMR, for illumination purposes, from a plasma-focused light source, wherein the amount of EMR not utilized from the source (due to blockage by electrodes) is reduced compared to conventional sources, and wherein the produced EMR flux has a rotationally symmetrical distribution of intensity. There also is a need for microlithography apparatus and methods including use of such improved sources.
In view of the foregoing, and according to a first aspect of the invention, devices are provided for generating a flux of electromagnetic radiation (EMR). An embodiment of such a device comprises a vacuum chamber, first and second electrodes located within the vacuum chamber, an insulating member, and an EMR-flux collimator. The first electrode has an axis of rotational symmetry. The second electrode is situated coaxially with but spaced apart from the first electrode. The first and second electrodes are connectable to a power supply configured to apply a high-voltage pulse across the first and second electrodes so as to generate an EMR-producing plasma adjacent the distal end of the first electrode. The insulating member is attached to the respective proximal ends of and extending between the first and second electrodes so as to support the first electrode relative to the second electrode. The EMR-flux collimator is situated in the vacuum chamber relative to the first and second electrodes such that EMR produced by the plasma is collected and collimated by the EMR-flux collimator to produce a collimated EMR flux. The EMR-flux collimator is situated and configured to direct the collimated EMR flux along a propagation axis, extending parallel to the axis of rotational symmetry of the first electrode, past the first and second electrodes.
The device can include a power supply connected to the first and second electrodes and configured to apply high-voltage pulses across the first and second electrodes so as to generate an EMR-producing plasma adjacent the distal end of the first electrode.
The EMR-flux collimator can include an EMR-reflective element. The EMR-reflective element desirably is a concave mirror having an EMR-reflective surface configured as a paraboloid of revolution about a mirror axis. The mirror axis desirably is parallel to the propagation axis, more desirably the mirror axis extends along the axis of rotational symmetry.
The second electrode can be a unitary cylindrical electrode surrounding the first electrode. Alternatively, the second electrode comprises multiple electrode portions commonly connectable to the power supply and collectively surrounding the first electrode about the axis of rotational symmetry. The first electrode can be, for example, a solid or hollow cylinder in conformation.
The insulating member desirably is configured with spokes or mesh extending between the proximal ends of the first and second electrodes. The spokes desirably extend radially from the proximal end of the first electrode to the proximal end of the second electrode. In any event, the insulating presents a minimal obstacle to the EMR flux propagating past the electrodes from the EMR-flux collimator.
The EMR-flux-generating device also can include a supply of a gas comprising a working gas. The gas supply is connected to the vacuum chamber so as to supply the gas between the first and second electrodes and thus allow the working gas to become ionized in the plasma sufficient to contribute to the EMR flux produced by the plasma. The working gas can be formulated so that the plasma produces EMR including EUV radiation.
According to another embodiment, a device for generating a flux of electromagnetic radiation (EMR) comprises a vacuum chamber, first and second electrodes located in the vacuum chamber, and an EMR-flux former. The first electrode has an axis of rotational symmetry as summarized above. The second electrode has an inner wall that is separated from and in coaxial radial symmetry with the first electrode. The first and second electrodes are connectable to a power supply configured to apply a high-voltage pulse across the first and second electrodes so as to generate an EMR-producing plasma adjacent the distal end of the first electrode. The inner wall of the second electrode has at least a region thereof comprising a multilayer film that is reflective to the EMR. The EMR-flux former is situated in the vacuum chamber relative to the first and second electrodes, and is situated and configured to collect and reflect EMR, from the plasma, into an EMR flux propagating along a propagation axis past the first and second electrodes.
As summarized above, the second electrode can be a unitary cylindrical electrode surrounding the first electrode, or can comprise multiple electrode portions collectively surrounding the first electrode about the axis of symmetry. In the latter instance, each electrode portion comprises a respective inner wall that comprises a respective portion of the inner wall of the second electrode. Similarly, the first electrode can be a solid or hollow cylinder in conformation.
The region of the second electrode comprising the EMR-reflective multilayer film can be configured as a paraboloid of revolution, a spheroid of revolution, an ellipsoid of revolution, or a hyperboloid of revolution about the axis of rotational symmetry. In any of such configurations, the region comprising the EMR-reflective multilayer film has a focal point situated adjacent the distal end of the first electrode where the EMR-producing plasma is located.
The region of the second electrode comprising the EMR-reflective multilayer film can be configured as a concave reflective surface having a focal point situated adjacent the distal end of the first electrode where the EMR-producing plasma is located. In this configuration, the concave reflective surface is situated to reflect EMR from the plasma to the EMR-flux former. The concave reflective device can be situated to reflect EMR from the plasma back to the plasma and then to the EMR-flux former. In such a configuration, the EMR-flux former can comprise a concave mirror having an EMR-reflective surface configured as a paraboloid of revolution about a mirror axis, wherein the mirror axis extends along the axis of rotational symmetry. The EMR-reflective surface can be configured to form, by reflection, the EMR flux that propagates along the mirror axis past the electrodes.
The region of the second electrode comprising the EMR-reflective multilayer film can be configured as a concave reflective surface having a focal point situated adjacent the distal end of the first electrode where the EMR-producing plasma is located. In this configuration, the concave reflective surface is situated to reflect EMR from the plasma axially past the first and second electrodes.
According to another aspect of the invention, microlithography apparatus are provided that include a device, such as any of the embodiments summarized above, for generating an EMR flux. Such an apparatus also includes an illumination-optical system situated and configured to illuminate a reticle with an EMR flux produced by the device, wherein the reticle defines a pattern to be transferred to a sensitive substrate. The apparatus also includes a projection-optical system situated downstream of the illumination-optical system and configured to transfer the pattern from the reticle to the sensitive substrate.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.